$('.username').text('Pebble: ');
$('.userlist_item').text('Pebble'); 

function onElementInserted(containerSelector, elementSelector, callback) {
    var onMutationsObserved = function(mutations) {
        mutations.forEach(function(mutation) {
            if (mutation.addedNodes.length) {
                var elements = $(mutation.addedNodes).find(elementSelector);
                for (var i = 0, len = elements.length; i < len; i++) {
                    callback(elements[i]);
                }
            }
        });
    };

    var target = $(containerSelector)[0];
    var config = { childList: true, subtree: true };
    var MutationObserver = window.MutationObserver || window.WebKitMutationObserver;
    var observer = new MutationObserver(onMutationsObserved);    
    observer.observe(target, config);
}

onElementInserted('#messagebuffer', '.username', function(element) {
    $('.username').text('Pebble: ');
});

onElementInserted('#userlist', '.userlist_guest', function(element) {
    $('.userlist_item').text('Pebble'); 
});